Electrolytic capacitors (e.g., tantalum capacitors) are increasingly being used in the design of circuits due to their volumetric efficiency, reliability, and process compatibility. For example, one type of capacitor that has been developed is a solid electrolytic capacitor that includes an anode (e.g., tantalum), a dielectric oxide film (e.g., tantalum pentoxide, Ta2O5) formed on the anode, a solid electrolyte layer, and a cathode. The solid electrolyte layer may be formed from a conductive polymer, such as described in U.S. Pat. No. 5,457,862 to Sakata, et al., U.S. Pat. No. 5,473,503 to Sakata, et al., U.S. Pat. No. 5,729,428 to Sakata, et al., and U.S. Pat. No. 5,812,367 to Kudoh, et al. The major drawback of the existing conductive polymer technology is its limited ability to produce high voltage capacitors, such as those having a rated voltage of more than 25V.
Various attempts have been made to address this problem. For example, U.S. Pat. No. 7,563,290 to Qiu, et al. describes a capacitor that contains a conductive polymer layer formed by dipping an anodized valve metal anode into a slurry of an intrinsically conductive polymer 1 to 15 times for a period of about 0.5 minute to 2 minutes to allow complete slurry coverage of its surface. Unfortunately, however, the present inventors have discovered that capacitors of this nature still tend to exhibit poor leakage current and equivalent series resistance (“ESR”) stability in the high humidity and/or high temperature environments associated with many commercial applications. Without intending to be limited by theory, the present inventors believe that when the anode body is fully immersed into a conductive slurry, gaseous bubbles can form in the polymer layer due to the presence of moisture from the slurry. The gaseous bubbles effectively become trapped within the fully applied polymer layer. Therefore, when they are evaporated during drying, they can actually cause portions of the polymer layer to tear away and leave behind inhomogeneities or “blisters” in the surface that reduce the ability of the layer to adhere to the anode body. Upon exposure to high humidity and/or temperature environments, these blisters can cause the layer to delaminate from the anode body, thereby reducing the degree of electrical contact and resulting in increased leakage current and ESR.
As such, a need currently exists for a solid electrolytic capacitor that contains a conductive polymer electrolyte, and which is capable of exhibiting good electrical performance in a wide variety of applications, including high voltage, humidity, and/or temperature environments.